Physicists propose new theories
of black holes from the very early
universe
'Primordial black holes,' believed to have formed shortly after the
Big Bang, might explain how gold, platinum and uranium are
created

UCLA physicists have proposed new theories for how the universe's first black
holes might have formed and the role they might play in the production of heavy
elements such as gold, platinum and uranium.
Two papers on their work were published in the journal Physical Review Letters.
A long-standing question in astrophysics is whether the universe's very first
black holes came into existence less than a second after the Big Bang or whether
they formed only millions of years later during the deaths of the earliest stars.
Alexander Kusenko, a UCLA professor of physics, and Eric Cotner, a UCLA
graduate student, developed a compellingly simple new theory suggesting that
black holes could have formed very shortly after the Big Bang, long before stars
began to shine. Astronomers have previously suggested that these so-called
primordial black holes could account for all or some of the universe's mysterious
dark matter and that they might have seeded the formation of supermassive
black holes that exist at the centers of galaxies. The new theory proposes that
primordial black holes might help create many of the heavier elements found in
nature.
The researchers began by considering that a uniform field of energy pervaded
the universe shortly after the Big Bang. Scientists expect that such fields existed
in the distant past. After the universe rapidly expanded, this energy field would

have separated into clumps. Gravity would cause these clumps to attract one
another and merge together. The UCLA researchers proposed that some small
fraction of these growing clumps became dense enough to become black holes.
Their hypothesis is fairly generic, Kusenko said, and it doesn't rely on what he
called the "unlikely coincidences" that underpin other theories explaining
primordial black holes.
The paper suggests that it's possible to search for these primordial black holes
using astronomical observations. One method involves measuring the very tiny
changes in a star's brightness that result from the gravitational effects of a
primordial black hole passing between Earth and that star. Earlier this year, U.S.
and Japanese astronomers published a paper on their discovery of one star in a
nearby galaxy that brightened and dimmed precisely as if a primordial black hole
was passing in front of it.

In a separate study, Kusenko, Volodymyr Takhistov, a UCLA postdoctoral
researcher, and George Fuller, a professor at UC San Diego, proposed that
primordial black holes might play an important role in the formation of heavy
elements such as gold, silver, platinum and uranium, which could be ongoing in
our galaxy and others.

The origin of those heavy elements has long been a mystery to researchers.
"Scientists know that these heavy elements exist, but they're not sure where
these elements are being formed," Kusenko said. "This has been really
embarrassing."
The UCLA research suggests that a primordial black hole occasionally collides
with a neutron star -- the city-sized, spinning remnant of a star that remains after
some supernova explosions -- and sinks into its depths.
When that happens, Kusenko said, the primordial black hole consumes the
neutron star from the inside, a process that takes about 10,000 years. As the
neutron star shrinks, it spins even faster, eventually causing small fragments to
detach and fly off. Those fragments of neutron-rich material may be the sites in
which neutrons fuse into heavier and heavier elements, Kusenko said.

However, the probability of a neutron star capturing a black hole is rather low,
said Kusenko, which is consistent with observations of only some galaxies being
enriched in heavy elements. The theory that primordial black holes collide with
neutron stars to create heavy elements also explains the observed lack of

neutron stars in the center of the Milky Way galaxy, a long-standing mystery in
astrophysics.
This winter, Kusenko and his colleagues will collaborate with scientists at
Princeton University on computer simulations of the heavy elements produced
by a neutron star-black hole interaction. By comparing the results of those
simulations with observations of heavy elements in nearby galaxies, the
researchers hope to determine whether primordial black holes are indeed
responsible for Earth's gold, platinum and uranium.

Story Source:
Materials provided by University of California - Los Angeles. Original written by
Katherine Kornei.